Scientists have pinned down exactly what it means to "fall in love." Researchers have found that an in-love brain looks very different from one experiencing mere lust, and it's also unlike a brain of someone in a long-term, committed relationship. Studies led by Helen Fisher, an anthropologist at Rutgers University and one of the leading experts on the biological basis of love, have revealed that the brain's "in love" phase is a unique and well-defined period of time, and there are 13 telltale signs that you're in it. 1. "This one's special" When you're in love, you begin to think your beloved is unique. The belief is coupled with an inability to feel romantic passion for anyone else. Fisher and her colleagues believe this single-mindedness results from elevated levels of central dopamine — a chemical involved in attention and focus — in your brain. 2. "She's perfect" People who are truly in love tend to focus on the positive qualities of their beloved, while overlooking his or her negative traits. They also focus on trivial events and objects that remind them of their loved one, day-dreaming about these precious little moments and mementos. This focused attention is also thought to result from elevated levels of central dopamine, as well as a spike in central norepinephrine, a chemical associated with increased memory in the presence of new stimuli. 3. "I'm a wreck!" As its well known, falling in love often leads to emotional and physiological instability. You bounce between exhilaration, euphoria, increased energy, sleeplessness, loss of appetite, trembling, a racing heart and accelerated breathing, as well as anxiety, panic and feelings of despair when your relationship suffers even the smallest setback. These mood swings parallel the behavior of drug addicts. And indeed, when in-love people are shown pictures of their loved ones, it fires up the same regions of the brain that activate when a drug addict takes a hit. Being in love, researchers say, is a form of addiction. 4. "Overcoming the challenge made us closer" Going through some sort of adversity with another person tends to intensify romantic attraction. Central dopamine may be responsible for this reaction, too, because research shows that when a reward is delayed, dopamine-producing neurons in the mid-brain region become more productive. 5. "I'm obsessed with him" People who are in love report that they spend, on average, more than 85 percent of their waking hours musing over their "love object." Intrusive thinking, as this form of obsessive behavior is called, may result from decreased levels of central serotonin in the brain, a condition that has been associated with obsessive behavior previously. (Obsessive-compulsive disorder is treated with serotonin-reuptake inhibitors.) 6. "I wish we could be together all the time" People in love regularly exhibit signs of emotional dependency on their relationship, including possessiveness, jealousy, fear of rejection, and separation anxiety. 7. "I hope we stay together forever" They also long for emotional union with their beloved, seeking out ways to get closer and day-dreaming about their future together. 8. "I'd do anything for her" People who are in love generally feel a powerful sense of empathy toward their beloved, feeling the other person's pain as their own and being willing to sacrifice anything for the other person. 9. "Would he like this outfit?" Falling in love is marked by a tendency to reorder your daily priorities and/or change your clothing, mannerisms, habits or values in order for them to better align with those of your beloved. 10. "Can we be exclusive?" Those who are deeply in love typically experience sexual desire for their beloved, but there are strong emotional strings attached: The longing for sex is coupled with possessiveness, a desire for sexual exclusivity, and extreme jealousy when the partner is suspected of infidelity. This possessiveness is thought to have evolved so that an in-love person will compel his or her partner to spurn other suitors, thereby insuring that the couple's courtship is not interrupted until conception has occurred. 11. "It's not about sex" While the desire for sexual union is important to people in love, the craving for emotional union takes precedence. A study found that 64 percent of people in love (the same percentage for both sexes) disagreed with the statement, “Sex is the most important part of my relationship with [my partner]." 12. "I feel out of control" Fisher and her colleagues found that individuals who report being "in love" commonly say their passion is involuntary and uncontrollable. 13. "The spark is gone" Unfortunately, being in love usually doesn't last forever. It's an impermanent state that either evolves into a long-term, codependent relationship that psychologists call "attachment," or it dissipates, and the relationship dissolves. If there are physical or social barriers inhibiting partners from seeing one another regularly — for example, if the relationship is long-distance — then the "in love" phase generally lasts longer than it would otherwise.